1. Field of the Invention
The present invention relates to a nonwoven fabric comprising a melt-stable, biodegradable lactide polymer composition and a process for manufacturing such nonwoven fabrics from a melt-stable, biodegradable lactide polymer.
2. Description of the Prior Art
The need for and uses of nonwoven fabrics have increased tremendously in recent years. Production of nonwoven roll goods was estimated at 2.5 billion pounds in 1992. Nonwoven fabrics are presently used for coverstock, interlinings, wipes, carrier sheets, furniture and bedding construction, filtration, apparel, insulation, oil cleanup products, cable insulating products, hospital drapes and gowns, battery separators, outerwear construction, diapers and feminine hygiene products.
There are basically three different manufacturing industries which make nonwovens; the textile, paper and extrusion industries. The textile industry garnets, cards or aerodynamically forms textile fibers into oriented webs. The paper industry employs technology for converting dry laid pulp and wet laid paper systems into nonwoven fabrics. The extrusion industry uses at least three methods of nonwoven manufacture, those being the spunbond, melt blown and porous film methods. The melt blown method involves extruding a thermoplastic resin through a needle thin die, exposing the extruded fiber to a jet of hot air and depositing the "blown" fiber on a conveyor belt. These fibers are randomly orientated to form a web. The spunbond method also utilizes a needle thin die, but orients or separates the fibers in some manner. The porous film method employs both slit and annular dies. In one method, a sheet is extruded and drawn, fibrillization occurs and a net-like fabric results.
A problem associated with current nonwoven materials is that recycling of the article containing the nonwoven fabric is generally not cost effective. In addition, disposal generally involves creating non-degradable waste. A vivid example is the disposal of diapers. Disposable diapers rely heavily on the use of nonwovens in their construction. Millions of diapers are disposed of each year. These disposable diapers end up in landfills or compost sites. The public is becoming increasingly alarmed over diapers that are not constructed of biodegradable material. In order to address the public's concern over the environment, diaper manufacturers are turning to biodegradable materials for use in their diapers. Currently, biodegradable materials made from starch based polymers, polycaprolactones, polyvinyl alcohols, and polyhydroxybutyrate-valerate-copolymers are under consideration for a variety of different uses in the disposable article market. However, to date, there has not been a satisfactory nonwoven fabric made from a biodegradable material which has properties that can withstand the present requirements of nonwoven fabrics.
Although not believed to be known as a precursor for nonwoven fabric, the use of lactic acid and lactide to manufacture a biodegradable polymer is known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. Processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatability in the final product. These processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield.
In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form may then be then sold to end users who convert, i.e., extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.
Prior to the present invention, it is believed that there has been no disclosure of a combination of composition control and melt stability requirements which will lead to the production of commercially viable lactide polymer nonwoven fabrics.
It is generally known that lactide polymers or poly(lactide) are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation which occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during processing which must be overcome.
Lactide polymer degradation at elevated temperature has been the subject of several studies, including: I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 267-285 (1985); I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 309-326 (1985); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 308-311 (1982); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 514-517 (1982); Ingo Luderwald, Dev. Polymer Degradation, vol. 2, pp. 77-98 (1979); Domenico Garozzo, Mario Giuffrida, and Giorgio Montaudo, Macromolecules, vol. 19, pp. 1643-1649 (1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada, Polymer, vol. 29, pp. 2229-2234 (1988).
It is known that lactide polymers exhibit an equilibrium relationship with lactide as represented by the reaction below: ##STR1##
No consensus has been reached as to what the primary degradation pathways are at elevated processing temperatures. One of the proposed reaction pathways includes the reaction of a hydroxyl end group in a "back-biting" reaction to form lactide. This equilibrium reaction is illustrated above. Other proposed reaction pathways include: reaction of the hydroxyl end group in a "back-biting" reaction to form cyclic oligomers, chain scission through hydrolysis of the ester bonds, an intramolecular beta-elimination reaction producing a new acid end group and an unsaturated carbon-carbon bond, and radical chain decomposition reactions. Regardless of the mechanism or mechanisms involved, the fact that substantial degradation occurs at elevated temperatures, such as those used by melt-processors, creates an obstacle to use of lactide polymers as a replacement for petrochemical-based polymers. It is apparent that degradation of the polymer during melt-processing must be reduced to a commercially acceptable rate while the polymer maintains the qualities of biodegradation or compostability which make it so desirable. It is believed this problem has not been addressed prior to the developments disclosed herein.
As indicated above, poly(lactide)s have been produced in the past, but primarily for use in medical devices. These polymers exhibit biodegradability, but also a more stringent requirement of being bioresorbable or biocompatible. As disclosed by M. Vert, Die Ingwandte Makromolekulare Chemie, vol. 166-167, pp. 155-168 (1989), "The use of additives is precluded because they can leach out easily in body fluids and then be recognized as toxic, or, at least, they can be the source of fast aging with loss of the properties which motivated their use. Therefore, it is much more suitable to achieve property adjustment through chemical or physical structure factors, even if aging is still a problem." Thus, work aimed at the bioresorbable or biocompatible market focused on poly(lactide) and blends which did not include any additives.
Other disclosures in the medical area include Nieuwenhuis (European Patent No. 0 314 245), Nieuwenhuis (U.S. Pat. No. 5,053,485), Eitenmuller (U.S. Pat. No. 5,108,399), Shinoda (U.S. Pat. No. 5,041,529), Fouty (Canadian Patent No. 808,731), Fouty (Canadian Patent No. 923,245), Schneider (Canadian Patent No. 863,673), and Nakamura et al., Bio. Materials and Clinical Applications, Vol. 7, p. 759 (1987). As disclosed in these references, in the high value, low volume medical specialty market, poly(lactide) or lactide polymers and copolymers can be given the required physical properties by generating lactide of very high purity by means of such methods as solvent extraction or recrystallization followed by polymerization. The polymer generated from this high purity lactide is a very high molecular weight product which will retain its physical properties even if substantial degradation occurs and the molecular weight drops significantly during processing. Also, the polymer may be precipitated from a solvent in order to remove residual monomer and catalysts. Each of these treatments add stability to the polymer, but clearly at a high cost which would not be feasible for lactide polymer compositions which are to be used to replace inexpensive petrochemical-based polymers in the manufacture of nonwoven products.
Furthermore, it is well-known that an increase in molecular weight generally results in an increase in a polymer's viscosity. A viscosity which is too high can prevent melt-processing of the polymer due to physical/mechanical limitations of the melt-processing equipment. Melt-processing of higher molecular weight polymers generally requires the use of increased temperatures to sufficiently reduce viscosity so that processing can proceed. However, there is an upper limit to temperatures used during processing. Increased temperatures increase degradation of the lactide polymer, as the previously-cited studies disclose.
Jamshidi et al., Polymer, Vol. 29, pp. 2229-2234 (1988) disclose that the glass transition temperature of a lactide polymer, T.sub.g, plateaus at about 57.degree. C. for poly(lactide) having a number average molecular weight of greater than 10,000. It is also disclosed that the melting point, T.sub.m, of poly (L-lactide) levels off at about 184.degree. C. for semi-crystalline lactide polymers having a number average molecular weight of about 70,000 or higher. This indicates that at a relatively low molecular weight, at least some physical properties of lactide polymers plateau and remain constant.
Sinclair et al. (U.S. Pat. No. 5,180,765) disclose the use of residual monomer, lactic acid or lactic acid oligomers to plasticize poly(lactide) polymers, with plasticizer levels of 2-60 percent. Loomis (U.S. Pat. No. 5,076,983) discloses a process for manufacturing a self-supporting film in which the oligomers of hydroxy acids are used as plasticizing agents. Loomis and Sinclair et al. disclose that the use of a plasticizer such as lactide or lactic acid is beneficial to produce more flexible materials which are considered to be preferable. Sinclair et al., however, disclose that residual monomer can deposit out on rollers during processing. Loomis also recognizes that excessive levels of plasticizer can cause unevenness in films and may separate and stick to and foul processing equipment. Thus, plasticizing as recommended, negatively impacts melt-processability in certain applications.
Accordingly, a need exists for a lactide polymer composition which is melt-stable under the elevated temperatures common to melt-processing resins in the manufacture of nonwoven fabrics. The needed melt-stable polymer composition must also exhibit sufficient compostability or degradability after its useful life as a nonwoven fabric. Further, the melt-stable polymer must be processable in existing melt-processing equipment, by exhibiting sufficiently low viscosities at melt-processing temperatures while polymer degradation and lactide formation remains below a point of substantial degradation and does not cause excessive fouling of processing equipment. Furthermore, the polymer lactide must retain its molecular weight, viscosity and other physical properties within commercially-acceptable levels through the nonwoven manufacturing process. It will be further appreciated that a need also exists for a process for manufacturing such nonwoven fabrics. The present invention addresses these needs as well as other problems associated with existing lactide polymer compositions and manufacturing processes. The present invention also offers further advantages over the prior art, and solves other problems associated therewith.